Phys Med Biol. 2013 Nov 21;58(22):8121-33. doi: 10.1088/0031-9155/58/22/8121.
High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane–parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%.
由直线加速器产生的高能电子束，通常在 6 至 20 MeV 范围内，用于局部浅表肿瘤的放射治疗中的小射野。由于其高空间分辨率，未屏蔽的硅二极管（Si-D）通常被认为是小电子野相对剂量测量的合适探测器。最近，一种新型合成单晶金刚石二极管（SCDD）显示出适合标准电子束和小光子束剂量学的特性。本研究的目的有两个：一是用直径为 2、3、4 和 5 cm 的商用管状施源器对 6 至 15 MeV 的小电子束进行成形，并评估新型 SCDD 剂量计在这种照射条件下的剂量性能，二是与市售的 Si-D 和平面平行电离室进行比较。测量了百分深度剂量曲线、射束轮廓和输出因子（OF）。在所有的测量中，所有剂量计之间都表现出很好的一致性。对于管状施源器，有两个主要的发现：（i）在一些射野和能量下，测量到大于 1 的 OF 值，最大可达 1.3 左右，这意味着在 10×10 cm2 参考射野的基础上，输出增加了 30%；（ii）对于每个直径的管状施源器，随着射束能量的增加，OF 值明显增加，在较小的施源器的情况下，增加了约 100%。这种 OF 行为与在这项研究中使用的具有相同尺寸和能量的小阻挡野典型观察到的行为有很大的不同。管状施源器的 OF 很大程度上取决于射野的大小，因此，对于这些施源器，插值法预测直线加速器的输出是不可取的，而像本工作中使用的硅和金刚石二极管这样的高分辨率探测器可以允许进行不确定度约为 1%的 OF 测量。
